Persons with Cystic Fibrosis (CF) are uniquely susceptible to respiratory infection with Pseudomonas aeruginosa and Staphlococcus aureus. In order to examine the immune response to these pathogens samples of saliva and serum were collected from CF patients and controls and assayed for antibodies reactive with these pathogens using a direct ELISA. Subjects with CF exhibited high antibody titers to Pseudomonas whereas control levels were low. The results thus far indicate that CF patients mount an immune response against these mucosal pathogens but are unable to eliminate them. An assay is under development which will permit the independent assay of secretory and serum IgA. This will enable the relative importance of local and serum derived IgA antibody to be determined.